JP2001230431A - Photoelectric conversion device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion device which optimizes the combination of the materials of a top cell and a bottom cell and realizes higher photoelectric conversion efficiency. SOLUTION: A photoelectric conversion device is provided with first and second p-n junctions. First p-n junction is formed in a semiconductor 4 shown by (Al1-yGay)1-xInxP and second p-n junction is formed in a semiconductor 2 which is substantially displayed by Ga1-z InzAs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion device for converting light energy into electric energy, and more particularly, to a photoelectric conversion device having improved photoelectric conversion efficiency for converting sunlight energy into electric energy when used in outer space. −
The present invention relates to a photoelectric conversion device using a group V compound semiconductor.

[0002]

2. Description of the Related Art In recent years, as a solar cell for space used for a power source of a spacecraft such as an artificial satellite, II, such as GaAs, has been used.
Examples of using multi-junction solar cells using an IV group compound semiconductor as a main material are increasing. These battery cells can be expected to have higher photoelectric conversion efficiency than silicon solar battery cells that have been widely used as space solar cells. For this reason, it is suitable for use in small satellites, high power satellites, and the like, which cannot be handled by silicon cells.

[0003] Battery cells currently most used as multi-junction solar cells are disclosed in, for example, US Patent Nos. 5,223,043 and 5,405,453. FIG. 20 shows the basic structure of these battery cells. The main material of this conventional multi-junction (two-junction) cell includes Ga _1-x In as a first battery cell (hereinafter referred to as “top cell”) 104 formed on the surface on the sunlight incident side. _x P is also a second battery cell which is formed below the top cell (hereinafter, referred to as "bottom cell") 102 GaAs is used, both of which are connected by a tunnel junction 103. The substrate 101 has GaAs
Alternatively, a Ge single crystal wafer is used. The composition ratio of Ga _1-x In _x P in the top cell is set to about x = 0.49 for the purpose of matching the lattice constant with GaAs in the bottom cell. In this case, the lattice constant of the material of the top cell and the material of the bottom cell is substantially equal to the lattice constant of Ge as the substrate, and is intended to be able to epitaxially grow on the Ge substrate relatively easily. At this time, the forbidden band width (band gap) Eg of the top cell is about 1.9 eV, and the band gap Eg of the bottom cell is about 1.4 eV.
V. The characteristics of this conventional multi-junction battery cell are about 26% at the laboratory level and about 22% at the industrial product level by a characteristic test using a light source simulating the solar spectrum in space.
% Of photoelectric conversion efficiency. Furthermore, in recent years,
In addition to the top cell and the bottom cell, a three-junction battery cell in which a pn junction is also formed on a Ge substrate has been developed.

[0004]

However, in order to cope with the dramatic progress of space development in recent years, the above-mentioned photoelectric conversion efficiency is insufficient, and a higher conversion efficiency is desired. The above-described conventional multi-junction battery cell has been historically developed as an advanced type of a GaAs solar cell formed on a Ge substrate, and as a result, the above-described structure has been determined. However, from the viewpoint of maximizing the use of solar energy, the above Ga _1-x
When considering the combination of In _x P and GaAs, this combination cannot be said to be the optimal combination as described below.

[0005] The theoretical photoelectric conversion efficiency of a solar cell comprising two pn junctions is, for example, IEEE Transaction on
Electron Devices, ED-34, p257. In this paper, the relationship between the expected value of the photoelectric conversion efficiency and the range of the band gap of the top cell and the bottom cell is shown by matching the band gap of the material of the top cell and the bottom cell with the incident light spectrum. When actually manufacturing a battery cell, usually, not only the band gap is a problem, but also the gap between the top cell and the bottom cell and between the bottom cell and the substrate in order to obtain a high-quality epitaxial growth layer. Lattice matching between them. FIG. 21 is a diagram showing a relationship between lattice constants and band gaps of various semiconductor materials. Based on the above-mentioned paper, FIG. 21 shows a band gap range U of the top cell and a band gap range L of the bottom cell which can obtain a conversion efficiency of 30% or more with respect to the solar spectrum (AMO) in outer space. . According to this figure, the combination of the materials of the conventional multi-junction battery cell described above, that is,
It can be seen that the photoelectric conversion efficiency does not reach 30% with the combination of Ga _1-x In _x P and GaAs.

[0006] Therefore, an object of the present invention is to provide a photoelectric conversion device capable of realizing higher photoelectric conversion efficiency by optimizing a combination of materials of a top cell and a bottom cell.

[0007]

According to a first aspect of the present invention, there is provided a photoelectric conversion device including first and second pn junctions, wherein the first pn junction is substantially (Al _{1) -y}
Ga _y ) _1-x In _x P is formed in the semiconductor represented by P, and the second pn junction is substantially formed in the semiconductor represented by Ga _1-z In _z As.

It has been said that it is important to satisfy at least the following conditions in order to achieve a high conversion efficiency of 30% or more. (A) The combination of the band gap energies of the material forming the top cell (hereinafter referred to as “top cell material”) and the material forming the bottom cell (hereinafter referred to as “bottom cell material”) is optimized. (B) Lattice matching between the top cell material and the bottom cell material. (C) To achieve lattice matching between the bottom cell material and the substrate material. (D) Match the thermal expansion coefficients of the growth layer material and the substrate material.

However, it is extremely difficult to find a combination of semiconductor materials that satisfies all of these conditions and can be manufactured economically. The present inventors have conducted extensive research on each of the above conditions. As a result, two conditions of (a) optimizing the combination of the band gap energies of the top cell material and the bottom cell material, and (b) achieving lattice matching between the top cell material and the bottom cell material are that the conversion efficiency is 30%. It was confirmed again that it is indispensable for realizing the above.

However, it is not necessary to emphasize the lattice matching between the bottom cell material and the substrate material. If the degree of lattice mismatch is within about 4%, good crystallinity can be obtained by devising a crystal growth technique. A layer can be formed ”(hereinafter, the condition obtained by relaxing the condition (c) is referred to as (c ′)).

[0011] Further, regarding the matching of the thermal expansion coefficient between the growth layer material and the substrate material, too much emphasis is not required.
If the thermal expansion coefficient of the growth layer is equal to or less than the thermal expansion coefficient of the substrate, crack propagation to the growth layer due to the difference in the thermal expansion coefficient can be suppressed. ”(Hereinafter, condition (d)) This condition, which was relaxed, is referred to as (d ')).

In the above investigation, the conditions (a),
Materials satisfying (b), (c ′) and (d ′)
The top cell contains (Al_1-yGa_y)_1-xIn_xBy P
And the bottom cell is Ga_1-z
In_zUse of semiconductor indicated by As
I was convinced that it was effective. These (Al
_1-yGa_y) _1-xIn_xP and Ga_1-zIn_zFind As
The above conditions (c ′) and
The policy of (d ') greatly contributes. Claim 1
By adopting the configuration, the above conditions (a), (b),
(C ') and (d') can all be satisfied
You. As a result, a photoelectric conversion device having a conversion efficiency of 30% or more is realized.
It is possible to manifest. In addition, in the indication of chemical composition
And substance B consisting of elements B, C, and P_{1- x}C_xIn P, the atom
C is a site occupied by C in the crystal lattice of the chemical formula CP
Occupies only x (≦ 1.0) and the remaining 1−x
The atom B occupies the site. The substance (A_1-yB_y)
_1-xC_xIn P, B above_1-xC_xB atom occupied by P
Of the sites that do, only y (≦ 1.0)
B atom occupies, and A atom occupies the remaining 1-y. The present invention
In the group III-V compound semiconductors targeted by InP, InP,
InAs, GaAs, GaP, etc. are usually zincblende (Zi
nc blende) type crystal structure. This sphalerite type crystal
The structure is the same as that of the diamond
It is similar to a land-type crystal structure.

According to a second aspect of the present invention, there is provided the photoelectric conversion device according to the first aspect, wherein the semiconductors Ga _{1 -z} In _z As and (A
l _1-y G _ay ) _1-x In _x P composition ratio z and x and y
Are respectively 0.11 <z <0.29, x = −0.346z ² + 1.08z +
0.484, and 131z ³ -66.0z ² + 9.17z + 0.309 <y <28.0z
³ -24.4z ² + 5.82z + 0.325.

With this configuration, it is possible to specifically optimize the band gap energy of the top cell material and the bottom cell material. The present inventors have proposed (Al _1-y G
in a _{y) 1- x} In _x P and Ga _1-z In _z As the purpose of obtaining an optimum composition, was calculated lattice constant and band gap energy for these semiconductors. FIG.
FIG. 4 is a diagram showing a band gap energy range A of a top cell material and a bottom cell material that can obtain a photoelectric conversion efficiency of 34% or more. In FIG. 1, the horizontal axis represents the band gap energy of the top cell material, and the vertical axis represents the band gap energy of the bottom cell material. In this figure, a region A where a conversion efficiency of 34% or more is expected is displayed as a region surrounded by one closed curve. In FIG.
The relationship between the bottom cell band gap and the top cell band gap when the bottom cell material is fixed and the top cell material is formed as a mixed crystal is shown as a line segment parallel to the horizontal axis. Since the top cell is a mixed crystal, a band gap also occurs within the range where the mixed crystal is established, and is displayed as a line segment. On the right side of each line segment, a two-component semiconductor material of a semiconductor material defined as a bottom cell and a mixed crystal formed thereon is shown. The bottom cell described there is G
The percentage of lattice mismatch with the lattice constant of e is shown. For example _{Ga .29 In .71 P-Al .30} In .70 P on
_{Ga .77 In .23 As (1.62%} > Ge) , the top cell is formed by a mixed crystal composed of _{Ga .29 In .71 P-Al .30} In .70 P, the bottom cell is Ga _.77 In _.23 A
s. The bottom cell _Ga.77I
This means that the lattice constant of _n.23 As is 1.62% larger than that of Ge. The composition range of (Al _1-y Ga _y ) _1-x In _x P and Ga _1-z In _z As constituting the top cell and the bottom cell respectively corresponding to the region A having the conversion efficiency of 34% or more is as follows. It is. z: The composition ratio z of Ga _1-z In _z As of the bottom cell material is
0.11 <z <0.29. _{x, y: (Al 1-} y Ga y) of the top cell material _1-x In x P
The composition ratios x and y of x are -0.346z ² + 1.08z + 0.484 and 131z ³ -66.0z ² + 9.17z + 0.309 <y <, respectively, assuming that z is within the above range. 28.0z ³ -24.4z ² +
It should be within the range of 5.82z + 0.325. The range of x is as shown in FIG. 2 according to the composition ratio z of Ga _1-z In _z As of the bottom cell material. The range of y is G of the bottom cell material.
It becomes as shown in FIG. 3 according to the composition ratio z of a _1-z In _z As.

If the composition ratios x, y and z of the top cell material and the bottom cell material are within the above ranges, it is expected that a conversion efficiency of 34% or more will be achieved. Further, x,
When y and z are within the above ranges, when the substrate is Ge, lattice mismatch with Ge can be kept within 2%. Regarding the coefficient of thermal expansion, Ge: 5.5 × 10 ⁻⁶ /
K, Ga _{1 -z} In _z As: 5.8 × 10 ⁻⁶ / K, and (Al
_{1-y Ga y) 1-} x In x P: as 4.8 × 10 ^-6 / K, since the three parties are close to each other, cracks may occur in the grown layer, cracks propagate in the growth layer No problem.

The present invention has a great feature in that a new material has been found for the semiconductor material constituting the solar cell by relaxing the condition of lattice matching with the substrate. That is, for the top cell, a new semiconductor was created based on the concept of mixed crystals. That is, although the crystal structure is the same, a new semiconductor was created by changing the elements constituting the crystal and changing the composition ratio of these elements. In general, it is well known that, in a compound semiconductor, mixed materials having the same crystal structure and different properties can be obtained by mixing different materials having the same physical properties such as lattice constant and band gap energy according to the mixing ratio. ing. Such a mixed crystal is formed by an LED (Light Emitting D
It has been put to practical use in devices such as iode) and laser diodes. In this case, the amount of the different materials to be mixed is not merely a degree of impurity doping but is accompanied by a composition change such that a change in crystal lattice constant or band gap occurs. Therefore, the above-described semiconductor G created based on the concept of mixed crystals
a _1-z In z As and _{(Al 1-y Ga y)} 1-x In x P is
It can be called a new semiconductor.

Here, the point that the concept based on the above-mentioned present invention differs from the above-mentioned US patent invention and the like, which is a conventional technique, will be described in detail. (1) Difference from U.S. Pat. No. 5,223,043 The above-mentioned U.S. Pat. No. 5,223,043 discloses the following three types of combinations as materials for a two-junction battery cell. (A) Combination of top cell Ga _x In _1-x P (0 <x <0.5) and bottom cell GaAs (B) Combination of top cell Ga _x In _1-x P (x = 0.51 ± 0.05) and bottom cell GaAs (C) top cell _{Ga x in 1-x P (} 0 <x <0.5) and bottom cell Ga _{x + 0.5} in _{0.5- x} as of a combination described above in combination with (0 <x <0.5), (a) and (B) is, as described above, combined on the premise that the growth layer is lattice-matched with the Ge substrate. On the other hand, the growth layer of the present invention does not need to be lattice-matched with the Ge substrate as shown in the above (c ′). Further, the material constituting the top cell of the photoelectric conversion device of the present invention is different from the material of the top cell of the photoelectric conversion device of the above-mentioned US patent invention. That is, A, which is a typical material of the top cell of the photoelectric conversion device of the present invention,
l _0.15 Ga _0.15 In _0.7 P contains 15% Al, and the above-mentioned US patent inventions (A), (B) and (C)
Is different from any of the above. That is, in the present invention, as shown in FIG. 1, the top cell contained Al (Al _1-y
By using Ga _y ) _1-x In _x P, the band gap between the top cell and the bottom cell can be set to an appropriate value, and high conversion efficiency can be achieved. In order to achieve this high conversion efficiency, it is essential to use a semiconductor material containing Al in a certain amount or more as a constituent material of the top cell. (2) Difference from U.S. Pat. No. 5,545,453 In the U.S. patent invention described above, the following two combinations are disclosed as materials for a two-junction battery cell. (D) Top cell (Ga, In) P (typically, G
a _0.49 In _0.51 P) and combination of bottom cell GaAs (E) Top cell (Al, In) P (typically A
l _0.55 In _0.45 P) Combination with Bottom Cell GaAs The above (D) and (E) are combinations based on the premise that they are lattice-matched with a Ge substrate. Different policies. The constituent materials of both the top cell and the bottom cell are different from the constituent materials of the present invention. (3) Others Others (Technical Digest of the In
ternational PVSEC-11, Sapporo, Hokkaido, Japan, 19
99, p593-594) disclose the following combinations. (F) Top cell In _0.49 Ga _0.51 P and bottom cell In
Combination with _0.01 Ga _0.99 P This combination of (F) is performed by adding 1% In to GaAs in order to correct a slight lattice mismatch between GaAs, which is a material of the conventional bottom cell, and the substrate Ge. In _0.01 G
a _0.99 P is lattice matched to the Ge substrate. Therefore, the combination of the above (F) also has a different basic design principle from the battery cell of the present invention. Further, both the top cell and the bottom cell are different from the present invention in the material constituting the battery cell.

The present invention does not require lattice matching with the substrate as compared with the prior arts (A) to (F) described above, and (Al _{1 -y} G _ay ) _{1 -x} In _x P Using a new material, the range of the composition ratio of the constituent materials is set for both the top cell and the bottom cell.

According to the third aspect of the present invention, there is provided a photoelectric conversion device according to the first aspect.
Or 2 conversion devices, (Al _1-yGa_y)_1-xIn_x
P and Ga_1-zIn_zAs is joined at the tunnel junction
ing.

The tunnel junction is highly doped p-type to electrically connect the top cell and the bottom cell.
⁺ n ⁺ junction included. According to this configuration, tandem conversion in which incident light having a high energy state is partially converted into electric energy in the top cell, and light energy reduced by an amount converted in the top cell in the bottom cell is partially converted into electric energy. It can be performed. Moreover, there is almost no loss of electric energy after being converted due to the tunnel junction. For this reason, it is possible to use electric energy with high conversion efficiency by the photoelectric conversion device.

According to the photoelectric conversion device of the fourth aspect, the first to fourth aspects are as follows.
3. The conversion device according to claim 3, further comprising a buffer layer between the growth layer including the first and second pn junctions formed on the substrate and the substrate, wherein the coefficient of thermal expansion of the buffer layer is It is equal to or higher than the thermal expansion coefficient of the growth layer immediately above the layer.

With this configuration, MOCVD (Metal Organ
ic Chemical Vapor Deposition)
Cracks can be contained only in the buffer layer. For this reason, crack generation in the growth layer and crack propagation to the growth layer can be suppressed.

According to a fifth aspect of the present invention, the thermal expansion coefficient of the substrate is smaller than that of the growth layer immediately above the buffer layer.

According to this configuration, a stronger crack suppressing effect by the buffer layer can be expected. The lattice constant of the buffer layer material is preferably close to the lattice constants of the growth layer and the substrate. Furthermore, as in the buffer layer of the sixth aspect, in the conversion device of the fourth or fifth aspect, it is desirable that the lattice constant of the conversion apparatus matches the lattice constant of the growth layer immediately above the buffer layer. The buffer layer material, specifically, for example, as a buffer layer according to claim 7, it is preferably composed substantially of _{GaAs 1-w Sb w (0.29} <w <0.33). The above composition ratio w is G
According to the value of the composition ratio z of a _1-z In _z As (0.11 <z <0.29), lattice matching is performed, or within a range of 0.29 <w <0.33 so as to be within a slight lattice mismatch range. Can be selected as appropriate.

In the photoelectric conversion device according to the eighth aspect, the first to fourth aspects are as follows.
7 wherein the first and second p
A growth layer including an n-junction is formed on any one of a GaAs single crystal substrate, a Ge single crystal substrate, and a Si single crystal substrate.

With this configuration, a growth layer having good crystallinity can be formed, and a photoelectric conversion device with high conversion efficiency can be easily provided. Further, by using a Ge single crystal for the substrate, lattice irregularity with the substrate can be suppressed to 2% or less. For this reason, not only can a high-quality epitaxially grown layer be obtained, but also a higher conversion efficiency can be obtained by forming a pn junction in Ge in the future.

According to the photoelectric conversion device of the ninth aspect, the first to the fifth aspects are as follows.
7, the growth layer including the first and second pn junctions formed on the substrate may be formed on the Si _1-x Ge _x mixed crystal layer provided on the Si single crystal substrate. Is formed.

With the above configuration, the degree of lattice irregularity can be suppressed, and a grown layer having excellent crystallinity can be formed. Further, a photoelectric conversion device with high conversion efficiency can be obtained using an inexpensive substrate.

[0029] According to the photoelectric conversion device of the tenth aspect, in the first aspect,
In any one of the converters of (1) to (9), a pn junction is further formed in an upper layer portion of the substrate on which the growth layers including the first and second pn junctions are formed.

According to this configuration, the light can be more effectively used, and the photoelectric conversion efficiency can be further improved.

[0031]

Next, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1) FIG. 4 is a sectional view showing the basic structure of a photoelectric conversion device according to Embodiment 1 of the present invention.
The light incident direction 10 is shown in the figure. In the following description, the light incident direction side of each layer is referred to as “front surface” or “upper layer” of each layer, and the opposite side is referred to as “back surface”. In the basic structure of FIG. 4, a bottom cell 2, a tunnel junction 3, and a top cell 4 are sequentially stacked on a substrate 1. For this lamination, a vapor phase growth method using organic metal (MOCVD method: Metal
Organics Chemical Vapor Deposition) and molecular beam epitaxy (MBE: Molecular Beam Epitaxy) are used. Although it is desirable to use a Ge single crystal for the substrate 1,
From the viewpoint of cost reduction, Si, or may be a mixed crystal such as Ge or Si _1-x Ge _x epitaxially grown on Si substrate "epitaxial substrate." Also, a pn junction may be provided in these substrates to form a three-junction structure, or a structure in which a potential barrier is formed by forming a hetero junction with the bottom cell material immediately above.

The bottom cell 2 has at least Ga _{1 -z} In _z A
It includes a junction between a p-layer and an n-layer made of a material having a composition of s (0.11 <z <0.29), that is, a pn junction. The pn junction may be sandwiched, for example, to provide a well-known window layer on the front side or a well-known back surface electric field layer on the back side to improve the carrier collection efficiency of the bottom cell. Further, a buffer layer for preventing the diffusion of the constituent elements and impurities from the substrate 1 may be provided.

The tunnel junction 3 is a heavily doped pn junction for electrically connecting the top cell and the bottom cell, and includes at least a pair of p ⁺ and n ⁺ layers. This p
A well-known device such as insertion of another pair of layers for suppressing impurity diffusion from the highly doped layer may be provided between the ⁺ layer and the n ⁺ layer. The material of the tunnel junction is Ga _{1 -z} In _z As or (Al ₁ -yG _ay ) _{1 -x} In _x
It may be P or a semiconductor material having another composition.

The top cell 4 has at least (Al _{1 -y} G
a _y ) A junction of a p-layer and an n-layer made of a material having a composition of _1-x In _x P is included. Here, x and y are between x = −0.346z ² + 1.08z + 0.484 and 131z ³ −66.0, respectively, between the In composition ratio z (0.11 <z <0.29) in the bottom cell.
z ² + 9.17z + 0.309 <have a relationship of ^{y <28.0z 3 -24.4z 2 + 5.82z} + 0.325. In the top cell 4, this pn
It goes without saying that, for example, a known window layer on the front surface side or a known back surface electric field layer or the like on the back surface may be provided to improve the carrier collection efficiency of the top cell with the junction interposed therebetween. .

FIG. 5 is a sectional view showing a specific example of a photoelectric conversion device manufactured based on the basic configuration of FIG. In FIG. 5, the bottom cell 2 includes an n-type window layer 21 and an n-type Ga
_1-z In _z As layer 22 and p-type Ga _1-z In _z As layer 23
And a p ⁺ type back surface electric field layer 24. The top cell 4 includes an n-type window layer 42 and an n-type (Al _1-y
Ga _y ) _1-x In _x P layer 43 and p-type (Al _1-y Ga _y ) _1-x I
An n _x P layer 44 and ap ⁺ -type back surface electric field layer 45 are provided. Further, antireflection films 81 and 82 and an n-type cap 41a are formed on the front surface side, and a front electrode 83 and a back electrode 84 for extracting electric energy are provided.

Next, a method of manufacturing the photoelectric conversion device shown in FIG. 5 will be described with reference to FIGS. In the present manufacturing method,
All the film forming processes and the like are continuously performed using a MOCVD apparatus. As the group III material, for example, an organic metal such as trimethylgallium, trimethylaluminum, and trimethylindium is supplied to the growth apparatus using hydrogen as a carrier gas. Group V materials include, for example, arsine (As
Gases such as H ₃ ), phosphine (PH ₃ ), and stibine (SbH ₃ ) are used. As the dopant of the p-type impurity or the n-type impurity, for example, diethyl zinc is used for p-type conversion, and monosilane (SiH ₄ ), disilane is used for n-type impurity.
(Si ₂ H ₆ ), hydrogen selenide (H ₂ Se) and the like are used. By supplying these material gases onto a substrate heated to, for example, 700 ° C., they are thermally decomposed, and a film of a desired compound semiconductor material can be epitaxially grown. The composition of these growth layers depends on the composition of the gas to be introduced.
Further, the film thickness can be controlled by the gas introduction time. FIG. 6 shows a p-type Ge substrate 1 on which a growth layer is formed.
FIG. First, as shown in FIG.
The bottom cell 2 is formed on the e-substrate 1 by MOCVD. The bottom cell 2 includes an n-type window layer 21 and an n-type G
a _{1 -z} In _z As layer 22, p-type Ga _{1 -z} In _z As layer 23,
And a p ⁺ type back surface electric field layer 24. The materials of the n-type window layer 21 and the p ⁺ -type backside electric field layer 24 can be appropriately selected from materials that lattice-match with the two Ga _1-z In _z As layers 22 and 23 in consideration of the conversion efficiency of the bottom cell. . Thus, for example, n-type as the n-type window layer _{21 (Al 1-y Ga y} ) 1-x In x P layer and p ⁺ -type as a back surface field layer 24 p ⁺ -type Ga _1-z In z As layer 24 Can be selected. Next, as shown in FIG.
A tunnel junction 3 is formed thereon. Tunnel junction 3 has p
⁺ -Type _{(Al 1-y Ga y)} 1-x In x P31 and the n ⁺ type (Al _1-y
Ga _y ) _{1- x} In _x P32. Next, as shown in FIG. 9, the top cell 4 is formed. This top cell 4
Are n-type cap layer 41, n-type window layer 42, n
Type _{(Al 1-y Ga y)} 1-x In x P layer 43 and the p-type (Al _1-y
Ga _y ) _1-x In _x P ₃₂ layer 44 and p ⁺ -type backside electric field layer 45
Consists of The n-type cap layer 4 is formed on the n-type window layer 42.
The reason why 1 is provided is to increase the ohmic contact of the n-electrode. n-type window layer 42 and p ⁺ -type backside electric field layer 45
Can be appropriately selected from materials that lattice-match with the (Al _1-y Ga _y ) _1-x In _x P ₃₁ layer in consideration of the conversion efficiency of the top cell. Therefore, for example, the n-type window layer 42
N-type (Al _1-y Ga _y ) with a lower In composition
The _1-x In x P layer, a p ⁺ -type back surface field layer 45 p ⁺
Type _{(Al 1 -y Ga y) 1} -x In x P layer is used. As the n-type cap layer 41, for example, an n ⁺ -type Ga _1-z In _z As layer can be used. Thereafter, the n-type cap layer 41 is selectively removed by etching to form an n-type cap 41a. In addition, for example, two layers of antireflection films 81 and 82 are formed on the surface of the top cell 41, and metal electrode films 83 and 84 are formed on the outermost surface and the back surface by a vacuum evaporation method or a sputtering method, as shown in FIG. The photoelectric conversion device is completed.

(Embodiment 2) FIG.
III-V compound semiconductor multi-junction solar cell
It is sectional drawing which shows another basic structure. This embodiment
Then, the buffer layer 5 is provided between the substrate 1 and the bottom cell 2.
Is provided. This buffer layer 5 has a lattice constant
Close to the lattice constant of the growth layer and substrate and its thermal expansion
The growth layer formed directly above the buffer layer,
The coefficient of thermal expansion of the material in the bottom layer of the
Composed of larger materials. This buffer layer 5
Is the heat between the substrate 1 and the growth layer when the temperature is lowered after the crystal growth.
Buffers cracks caused by differences in expansion coefficients
Cracks in the growth layer
It is intended to prevent crack propagation to long layers. this
Therefore, the bottom cell 2, the tunnel junction 3, and the top cell 4
Is unaffected by cracks and is protected from cracks.
You. With regard to this buffer layer, more desirably,
Thermal expansion coefficient of material of growth layer formed directly on buffer layer
Is preferably larger than the coefficient of thermal expansion of the substrate.
As a specific material of the buffer layer, for example, a bottom cell
Material Ga _1-zIn_zAs composition range 0.11 <z <0.29
Accordingly, Ga is appropriately selected as w._1-wAs_wSb (0.29 <w <
0.33). Bottom cell 2, tunnel junction 3 and
The cell 4 is the same as in the first embodiment.

FIG. 11 shows a specific photoelectric conversion device based on the basic structure of the photoelectric conversion device shown in FIG. The contents of the bottom cell 2 and the top cell 4 having a multilayer structure, the antireflection films 81 and 82, and the electrodes 83 and 84 are the same as those described in the first embodiment.

The present invention relates to a basic material selection in which a multilayer growth layer including a top cell, a tunnel junction and a bottom cell is formed on a substrate to provide a photoelectric conversion device with high conversion efficiency. . Therefore, a tunnel junction is separately provided between the buffer layer and the bottom cell,
It goes without saying that inserting another layer such as a strain relaxation layer between the top cell and the tunnel junction or between the tunnel junction and the bottom cell is also included in the scope of the present invention. Further, whether the light receiving surface side is p-type or n-type is also included in the scope of the present invention as long as the material selection falls under the present invention.

By providing the buffer layer as in the present embodiment, cracks which may be generated due to differences in thermal expansion of the respective parts of the photoelectric conversion device when the temperature is lowered after the film forming process are confined in the buffer layer. I can put it. Therefore, the production yield is improved, and the production cost can be reduced. In addition, by adjusting the lattice constant, the growth layer with excellent crystallinity can be formed by lattice matching with the growth layer immediately above, and the photoelectric conversion efficiency can be improved.

(Embodiment 3) FIG. 12 shows Embodiment 3 of the present invention.
FIG. 2 is a cross-sectional view illustrating a basic configuration of the photoelectric conversion device in FIG. This
The photoelectric conversion device of (1) also forms a pn junction on the surface of the substrate,
And a tunnel junction between the buffer layer 5 and the bottom cell 2
9 is further provided. Basic configuration shown in FIG.
Section of a specific example of a photoelectric conversion device actually configured based on
The figure is shown in FIG. Manufacturing of the photoelectric conversion device shown in FIG.
The method will be described with reference to FIGS. FIG.
Reference numeral 4 denotes a p-type Ge substrate 1. Form a cell structure on this
However, during the epitaxial growth during that, As
By diffusion, a thin n is formed on the p-type Ge substrate 12.
The mold layer 11 can be formed. Next, as shown in FIG.
As described above, the buffer layer 5 is formed on the substrate 1 by the MOCVD method.
Form. As a material of this buffer layer, for example, n
Type GaAs_1-wSb_w(0.29 <w <0.33)
Wear. Next, as shown in FIG.
Form a new one. As the tunnel junction 9, for example,
If p⁺Type Ga_1-zIn_zAs91 and n⁺Type Ga _1-zIn_zA
s92. Next, FIG.
Thus, the bottom cell 2 is formed. The bottom cell 2
An n-type window layer 21 and an n-type Ga _1-zIn_zAs layer 2
2 and p-type Ga_1-zIn_zAs layer 23 and p⁺Mold back surface electric field
Consists of layer 24. n-type window layer 21 and p⁺Mold back surface electric field layer
24 is made of Ga_1-zIn_zAs layers 22, 23 and lattice alignment
Considering the conversion efficiency of the bottom cell
You can choose any. For example, the n-type window layer 21 is an n-type window layer.
(Al_1-yGa_y)_1-xIn_xP and p⁺Mold back surface electric field layer
24 is p⁺Type Ga_1-zIn_zAs. Next
Next, as shown in FIG. 18, a tunnel junction 3 is formed.
This tunnel junction 3 is p⁺Mold (Al_1-yGa_y)_1-xIn_xP
Layers 31 and n⁺Mold (Al_1-yGa_y)_1-xIn_xP layer 32
Become. Next, as shown in FIG.
To achieve. The top cell 4 includes, in order from the top, an n-type window layer 4.
2 and n-type (Al_1-yGa_y)_1-xIn_xP layer 43 and p-type
(Al_1-yGa_y)_1-xIn_xP layer 44 and p⁺Mold back surface electric field layer
45. Further, in the present embodiment, n
N-type cap layer 4 to enhance ohmic contact of the electrode
1 is provided. n-type window layer 42 and p⁺Mold back side
The material of the boundary layer 45 is (Al_1-yGa_y)_1-xIn_xP layer 43,
Conversion efficiency of top cell from materials that lattice-match with 44
Can be appropriately selected in consideration of the above. n-type window layer 42
For example, for example, an n-type (Al_1-yGa
_y)_1-xIn_xP layer and p⁺Mold back surface electric field layer 45
Is, for example, p⁺Mold (Al_1-yGa_y)_1-xIn_xUsing the P layer
Can be The n-type cap layer 41 includes, for example, an n-type
Ga_1-zIn_zAs can be used. Then,
The top layer 41 is selectively etched and removed, and the top layer 41 is removed.
On the back surface of the capsule, for example, two layers of antireflection films 81 and 82 are provided.
It is formed. Finally, a metal electrode film 8 is formed on the top and bottom surfaces.
3,84 formed by vacuum deposition or sputtering
Thus, the photoelectric conversion device shown in FIG. 13 is completed.

The above-mentioned photoelectric conversion device is formed using an unprecedented new semiconductor material for the purpose of providing the highest photoelectric conversion efficiency by the combination of the band gap of the top cell and the bottom cell. For this reason, the conventional I
It is possible to achieve a conversion efficiency that greatly exceeds the conversion efficiency of a photoelectric conversion device using a II-V group semiconductor.

Although the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. Not limited. The scope of the present invention is shown by the description of the claims, and further includes all modifications within the meaning and scope equivalent to the description of the claims.

[0045]

The semiconductor multi-junction solar cell containing the group III-V compound according to the present invention has a structure in which the conversion efficiency is 34% or more in theory, and the conversion efficiency is 27% or more even at the industrial product level. Is obtained. Therefore, it is used for power generation equipment for small satellites and large power sanitation, and is expected to contribute to the development of communication-related related industrial fields.

Although the embodiments of the present invention have been described above, the embodiments of the present invention disclosed above are merely examples, and the scope of the present invention is not limited to these embodiments. Not limited. The scope of the present invention is shown by the description of the claims, and further includes all modifications within the meaning and scope equivalent to the description of the claims.

[Brief description of the drawings]

FIG. 1 is a top cell (Al _1-y Ga _y ) of the present invention.
Is a diagram illustrating the respective optimum composition range of _1-x In x P and and the bottom cell Ga _1-z In z As.

FIG. 2 is a diagram showing a range of a composition ratio x in a top cell (Al _1-y Ga _y ) _1-x In _x P according to a composition ratio z of a bottom cell Ga _1-z In _z As.

FIG. 3 is a diagram showing a range of a composition ratio y in a top cell (Al _1-y Ga _y ) _1-x In _x P according to a composition ratio z of a bottom cell Ga _1-z In _z As.

FIG. 4 is a cross-sectional view showing a basic structure of a group III-V compound semiconductor multi-junction solar cell according to the first embodiment.

FIG. 5 is a cross-sectional view of a specific III-V compound semiconductor multi-junction solar cell according to the first embodiment.

6 is a cross-sectional view of the substrate of the multi-junction solar cell of FIG.

FIG. 7 is a cross-sectional view at the stage when a bottom cell is formed on the substrate shown in FIG.

8 is a cross-sectional view of a state where a tunnel junction is formed in the state of FIG. 7;

FIG. 9 is a cross-sectional view of a state where a top cell is formed in the state of FIG. 8;

FIG. 10 is a cross-sectional view showing a basic structure of a group III-V compound semiconductor multi-junction solar cell in Embodiment 2.

FIG. 11 is a specific III-V according to the second embodiment.
1 is a cross-sectional view of a multi-junction type solar cell made of a group III compound semiconductor.

12 is a cross-sectional view showing a basic structure of a group III-V compound semiconductor multi-junction solar cell in Embodiment 3. FIG.

FIG. 13 is a specific III-V according to the third embodiment.
1 is a cross-sectional view of a multi-junction type solar cell made of a group III compound semiconductor.

14 is a cross-sectional view of a stage in which an n-type impurity is introduced into an upper portion of a p-type substrate of the multi-junction solar cell of FIG.

FIG. 15 is a cross-sectional view of a state where a buffer layer is formed in the state of FIG. 14;

16 is a cross-sectional view of a state where a tunnel junction is formed in the state of FIG.

FIG. 17 is a cross-sectional view of a state where a bottom cell is formed in the state of FIG. 16;

FIG. 18 is a cross-sectional view of a state where a tunnel junction is formed in the state of FIG. 17;

FIG. 19 is a cross-sectional view of a state where a top cell is formed in the state of FIG. 18;

FIG. 20 is a cross-sectional view showing the basic structure of a conventional III-V compound semiconductor multi-junction solar cell.

FIG. 21 is a diagram showing a relationship between lattice constants and band gap energies of various semiconductors.

[Explanation of symbols]

 1 substrate, 2 bottom cell, 3 tunnel junction, 4 ton
Cell, 5 buffer layer, 81, 82 anti-reflection film,
83,84 electrode metal, 9 tunnel junction, 11 n-type
Ge layer, 12 p-type Ge substrate, 21 n-type window layer (n-type
(Al_1-yGa_y) _1-xIn_xP layer), 22 n-type Ga_1-z
In_zAs layer, 23 p-type Ga_1-zIn_zAs layer, 24
p⁺Mold back surface electric field layer (p⁺Type Ga_1-zIn_zAs layer), 31
p⁺Type (Al_{1- y}Ga_y)_1-xIn_xP layer, 32 n⁺Type (A
l_1-yGa_y)_1-xIn_xP layer, 41 n-type cap layer (n⁺
Type Ga_1-zIn_zAs layer), 41a n-type cap, 42
 n-type window layer (n-type (Al_1-yGa_y)_1-xI
n_xP layer), 43 n-type (Al_{1 -y}Ga_y)_1-xIn_xP
Layer, 44 p-type (Al_1-yGa_y)_1-xIn_xP layer, 45
p⁺Mold back surface electric field layer (p⁺Type (Al_1-yGa_y)_1-xIn_xP
Layer), 91 p⁺Type Ga_1-zIn _zAs layer, 92 n⁺Type G
a_1-zIn_zAs layer, A region with a conversion efficiency of 34% or more,
U Top cell bandgear to achieve 30% conversion efficiency
Gap, L-conversion efficiency 30%
Band gap range.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yuji Komatsu 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Masafumi Shimizu 22-22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka F-term (for reference) 5F051 AA08 BA02 CA14 DA03 DA17 DA19 DA20 GA04 HA03

Claims (10)

[Claims] 1. A photoelectric conversion device comprising first and second pn junctions, wherein the first pn junction is substantially (Al
_1-y Ga _y ) formed in the semiconductor represented by _1-x In _x P, and the second pn junction is formed substantially in the semiconductor represented by Ga _1-z In _z As; Photoelectric conversion device. 2. The semiconductor according to claim 1, wherein said Ga _1-z In _z As and (A
l _1-y G _ay ) _1-x In _x P composition ratio z and x and y
Are respectively 0.11 <z <0.29, x = −0.346z ² + 1.08z +
0.484, and 131z ³ -66.0z ² + 9.17z + 0.309 <y <28.0z
³ -24.4Z in the range of ² + ^5.82z + 0.325, the photoelectric conversion device according to claim 1. Wherein the _{(Al 1-y Ga y)} 1-x In x P and Ga _1-z In z As are joined in the tunnel junction,
The photoelectric conversion device according to claim 1. 4. A buffer layer between a growth layer including the first and second pn junctions formed on a substrate and the substrate, wherein the buffer layer has a thermal expansion coefficient of The photoelectric conversion device according to any one of claims 1 to 3, which has a thermal expansion coefficient equal to or higher than a thermal expansion coefficient of a growth layer immediately above the growth layer. 5. The photoelectric conversion device according to claim 4, wherein a thermal expansion coefficient of the substrate is smaller than a thermal expansion coefficient of a growth layer immediately above the buffer layer. 6. The photoelectric conversion device according to claim 4, wherein a lattice constant of the buffer layer matches a lattice constant of a growth layer immediately above the buffer layer. 7. The method according to claim 7, wherein the buffer layer is substantially GaAs _{1-w
Sb w (0.29 <w <0.33} ) are made of a material that is displayed by the photoelectric conversion device according to claim 4-6. 8. A growth layer including the first and second pn junctions comprises a GaAs single crystal substrate, a Ge single crystal substrate,
The photoelectric conversion device according to claim 1, wherein the photoelectric conversion device is formed on any one of the i-single-crystal substrates. 9. A growth layer including the first and second pn junctions formed on a substrate is formed on a Si _1-x Ge _x mixed crystal layer formed on a Si single crystal substrate. The photoelectric conversion device according to claim 1, wherein: 10. The photoelectric conversion device according to claim 1, wherein a pn junction is further formed in an upper layer portion of the substrate on which the growth layers including the first and second pn junctions are formed. Conversion device.